The oil spewing into the Gulf of Mexico was coming out at least twice as fast as officials said initially, government officials said Thursday, meaning that it has become the largest U.S. oil spill.

In a news conference Thursday morning, U.S. Geological Survey Director Marcia McNutt said that the oil was coming out at a rate of 12,000 to 19,000 barrels a day. Originally, officials from the Coast Guard and BP had estimated the flow at 5,000 barrels a day.

The flow rate measurements were taken before the attempt at a "top kill" of the leaking well began Wednesday. That effort, which flooded the well with high-pressure mud, has made it difficult to determine how much oil is leaking out now.

But McNutt's estimate made it clear that this spill is larger than the nation's previous worst spill, the 1989 Exxon Valdez disaster. That incident, which has generally been reported in gallons as opposed to barrels, dumped 11 million gallons of oil into Alaska's Prince William Sound. That is about 257,000 barrels. (A barrel contains about 42 gallons of oil.)

Using the low end of the new estimate -- 12,000 barrels a day -- this spill is larger than the Valdez disaster. Using the high end, it is more than twice as large.

A spokesman for the Interior Department, Frank Quimby, said a team of scientists had calculated the new estimates using three techniques. One used video of the plume of oil escaping from the pipe and then used computer modeling to come up with an estimate of 12,000 to 25,000 barrels a day.

Another relied on a NASA plane, which flew over the gulf using equipment designed to tell oil from water on the surface. Its analysis yielded an estimate of 12,000 to 19,000 barrels a day.

A third method relied on measurements from a tube stuck into a leaking pipe, which has siphoned some of the leaking oil to the surface. By analyzing that flow, the team of scientists estimated the flow was 12,000 barrels a day.

Quimby said the final estimate was made by combining those three. He said the team had not determined whether the rate had increased or decreased over time.

"This is the current rate, and it's probably been the average rate for a month," Quimby said, meaning before the top kill began.